Bone Health in Highly Trained Female Athletes

A Review of the Current State of Knowledge

Highly trained female athletes are often at peak cardiovascular fitness but face important threats to their skeletal health. Women that train intensively may produce abnormally low levels of estrogen, which in turn, may lead to weakened bones. Low bone strength (or osteopenia), is a risk factor for stress fractures. Young adults with osteopenia are also more likely to develop osteoporosis later in life.

It is generally accepted that exercise promotes bone health. However, research focusing on the relationship between intensive exercise, bone health, and estrogen produce alarming results concerning the health of female athletes. The hormone estrogen is responsible for growth and development of reproductive organs, as well as onset and regulation of menstruation. In addition, estrogen is essential for maintaining bone health in women. Events that result in rapid declines in a woman's estrogen level, such as menopause and ovariectomy (removal of the ovaries), also result in rapid losses in her bone mass and bone strength

Regular vigorous exercise is associated with decreased estrogen levels in the blood. In one study, healthy women who began training for a marathon reduced their estrogen levels by over 50%. These low estrogen levels often result in menstrual irregularity in a large proportion of intensively training athletes. Irregularities can include a late onset of menstrual periods, infrequent periods (oligomenorrhea), absent periods (amenhorrea), or more subtle abnormalities, such as a shortened luteal phase and anovulatory cycles. (The luteal phase refers to the phase of menstruation during which progesterone is released from the ovum and the uterine lining proliferates; anovulatory cycles are those menstrual cycles in which a woman does not ovulate properly. These abnormalities can only be detected by specific medical tests.) A recent survey of competitive collegiate cross-country runners found that 56% missed several menstrual periods a year or had no periods at all. Cumulative incidence of amenorrhea (loss of period) and oligomenorrhea among all athletes is even higher. Most studies have considered athletes with infrequent or absent periods and have not evaluated athletes with more subtle menstrual disturbances. However, one study found that runners who menstruate monthly but who have anovulatory cycles and/or shortened luteal phases also lose bone. This study is of particular interest because it demonstrates that highly training females who appear to be menstruating normally may still be at risk for osteopenia.

The cause of estrogen-deficiency and menstrual irregularity in athletes is not known with certainty.However, studies have identified these risk factors: earlier onset of training, more intense training, psychological stress, nutritional inadequacy, low body weight, low body mass, and changes in body composition.

Because exercise is normally beneficial to the bone, it has been hypothesized that women athletes may escape bone loss even if they develop menstrual irregularities. However, studies show that this is not the case. Nineteen studies have been conducted in which the bone strength (measured as bone density) of normally menstruating women was compared to the bone strength of athletes with irregular or absent periods. Fifteen of these studies showed that bone density was significantly lower in amenorrheic or oligomenorrheic athletes. Three of the four remaining studies showed a trend towards lower bone density in amenorrheic/oligomenorrheic athletes, but this was not statistically significant.

Women in late adolescence and early adulthood should still be building bone. Studies that measure athletes at a single time point cannot tell us if osteopenic athletes simply stop building bone or if they also breakdown existing bone. However, three studies have been conducted in which amnorrheic/oligomenorrheic athletes were measured yearly. These studies found amenorrheic athletes in their late teens and early twenties not only fail to gain, but actually begin to lose bone. The magnitude of bone loss is serious. Amenorrheic/oliomenorrheic athletes have 8 to 31% lower bone density than normally menstruating athletes, and 3 to 24% lower bone density than non-exercising, normally menstruating controls. Alarmingly, many of these women have extremely low bone mass; some have bone densities comparable to women in their 70's and 80's.

It is likely that poor nutrition and disordered eating add to the risk of bone health problems in female athletes. Low body weight and a lean physique are desirable attributes in many sports, such as gymnastics, dancing, figure skating, and running. Athletes striving to excel in these sports may develop disordered patterns that contribute to both their menstrual irregularities and to the weakened state of their bones. Proper nutrition is essential for normal menstruation and for healthy bone; non-athletic women who are malnourished, such as women with anorexia nervosa will develop amenorrhea, osteopenia and even osteoporosis. The combination of disordered eating, amenorrhea, and osteoporosis has been termed the "female athlete triad."

Stress fractures often plague young female athletes. In surveying competitive collegiate cross-country runners, 44% had experienced at least one stress fracture and 21% had suffered multiple stress fractures. Several studies report an association between current menstrual irregularity and stress fracture incidence among female athletes, which indicates that stress fractures may be related to estrogen deficiency and menstrual irregularity. Two studies found that a history of menstrual irregularity is a risk factor for stress fractures. Stress fracture risk was 2 to 4 times greater for amenorrheic/oligomenorrheic athletes than for normally menstruating.

Few studies have evaluated long term outcomes for athletes with menstrual irregularities and low bone density.Three studies that followed amenorreic athletes over time found that some recovery of bone mass can occur; athletes who gained weight, decreased training, and resumed menses (concomitant with increased estrogen levels) had bone density gains of 3 to 9% over the first year. However, formerly amenorrheic athletes still had significantly lower bone density compared to controls, suggesting that bone health may be permanently compromised if intervention is initiated too late. The longest running study to follow previously amenorrheic/oligomenorrheic athletes lasted eight years. Despite the return of regular menstrual periods for several years, the bone density of formerly amenorrheic/oligomenorrheic athletes remained 15% less than the bone density of athletes who had never been amenorrheic. This suggests that some bone loss may be irreversible and that early intervention is crucial.

Several studies show that the lower an athlete's estrogen level falls and the longer that her menstrual irregularity persists, the greater the deficits in her bone.These observations suggest a direct relationship between a low estrogen state and bone loss. In theory then the establishment of normal estrogen levels should prevent and even reverse bone loss. Oral contraceptives provide estrogen and regulate the menstrual cycle; thus, it has been hypothesized that oral contraceptives can be used to strengthen the bones of women athletes. In addition, oral contraceptives may be protective against stress fractures. Four retrospective studies found that female athletes who took oral contraceptives had suffered 2 to 4 times fewer stress fractures than non users. These studies suggest that oral contraceptive use reduces stress fracture incidence. However, these studies cannot establish a cause and effect relationship. For example, the women who chose to take oral contraceptives may have had greater bone mass before they started taking oral contraceptives

To test these hypotheses, the Bone Health in Female Runners Intervention Trial, or B-FIT, is being conducted at the Stanford School of Medicine. This is a multi-site, randomized trial in which female long distance runners are assigned to either the treatment group or the control group. Treatment athletes are administered oral contraception; control athletes do not take oral contraception for the remainder of the study. These athletes are followed for two years during which their bone density is measured three times and their training and nutritional habits are examined every six months.This study is currently seeking more participants from the New York area and California. If you are interested please contact the B-FIT office at 1-877-RUN-BFIT or [email protected].